How to troubleshoot my air conditioning unit for Nissan March?

SMILIYEVANGELISTA

1997 NISSAN MARCH

30,000 MILES

My air condition become watery when I run my car 100 km/h and become warm after that I just off it and open it again after 30 min it produce cloudy air that is not anymore much cold. Please give me a idea on how can I fix it by my self. Thank you so much!